You can use

MyRange.Information(wdVerticalPositionRelativeToPage)

to retrieve the vertical position of the start of range. If you know the
vertical position of a range at start of page (ie, if the layout is fixed
you can work this out in advance) then simply compare with that. Or get the
vertical position of the preceding character and compare ... if it's less,
the preceding character is on the same page, and your range is therefore not
at the start --

if activedocument.Range(MyRange.Start - 1, MyRange.Start -
1).Information(wdVerticalPositionRelativeToPage) <
MyRange.Information(wdVerticalPositionRelativeToPage) then
       ... not at start of page

Bear in mind, this will throw an error if MyRange is at the start of
document.

Jeremy was telling us:
Jeremy nous racontait que :

You could compare the currently selected selection range.start value with
the current page range.start value:

'_______________________________________
Dim curPageRge As Range
Dim curSelRge As Range

Set curSelRge = Selection.Range

curSelRge.Collapse wdCollapseStart

Set curPageRge = curSelRge.Bookmarks("\page").Range

If curSelRge.Start = curPageRge.Start Then
   MsgBox "Selection is at top of page."
Else
   MsgBox "Selection is NOT at top of page."
End If
'_______________________________________